Caldera faults

A caldera is a large circular hollow, that can have a diameter ranging from one
kilometer to over 25 kilometers, and are usually formed by the collapse of surficial
rocks into an emptied or unsupported magma chamber. A result of these collapsing
rocks are a series of concentric normal faults, also known as ring faults. The faults do
not form a continuous ring around the caldera, but displacement from one fault
overlaps with the next, forming a circular pattern (Fig. 1). The faults are terminated by
the chamber boundary, therefore displacement only occurs in the rock layers above
the chamber. Calderas can become highly fragmented if collapse occurs in
successive volcanic events. Ring faults are not just associated with calderas, but can
occur whenever there is a pocket or hole below the surface where rocks can cave into
it. An example of a caldera is Crater Lake on Mount Mazama in Oregon, whose
diameter is 9 km.

Another structural boundary of calderas are indicated by ring dikes. Ring dikes
are subsurface concentric dikes, where magma usually fills existing fractures. Ring
dikes are a subsurface expression of the circular pattern occurring at the surface.
They tend to dip vertically or outwards at high angles. In some cases, the fractures
due to faulting act as conduits for the magma, not only for dikes, but for a future
volcanic event.

References cited

Branney, M.J., and Kikelaar, P. “Volcanotectonic faulting, soft-state deformation,
and rheomorphism of tuffs during development of a piecemeal caldera, English Lake
District.” Geol. Soc. of Am. Bul. 106:507-530. 1994.